The MAX11209 can do 18 bits at sample rates up to 120 Hz (true) or 480 Hz (4x oversampled). It has both external analog and internal digital scaling. There is +Vref and -Vref, as well as +Vin and -Vin, so you can set your own full-range scale and offset via analog voltages (many ADCs have only +Vref, so you can’t set your own analog offset above ground). Maximum Vin = 3.6 V. The data sheet claim that CS/ can be tied low is apparently false (must be brought high after each SPI transaction).
The 11209 is part of a family, some other variants do 24 bits (although there I think some bits will be just noise). With the 11209 I can so far confirm that the input noise is less than 2 uV at a 15 Hz rate. Datasheet claims 0.57 uV RMS at 10 Hz.
Via the forum.